Authors

Document Type

Technical Report

Publication Date

12-1990

Abstract

The Lake Mead Fertilization Project is a research program designed to investigate the potential for using large-scale artificial fertilization to enhance the game fisheries of this reservoir through an increase in the population of threadfin shad, the system's primary forage species. A substantial decline in the population of largemouth bass, together with poor condition of adult striped bass, are the two major issues affecting the Lake Mead game fisheries. Both issues have been hypothesized to be a result of an inadequate amount of forage in the reservoir. Previous studies have in turn suggested that a major factor limiting the shad population may be low productivity levels at the base of the food chain. Approximately 20,000 gallons of liquid ammonium polyphosphate were applied to about 20,000 acres of the Overton Arm in each of the years 1987 through 1989. The fertilizer was applied in late May or early June. Approximately 300 volunteer boats were used to distribute fertilizer in the early stages of the program; these were subsequently replaced with barge applications. A series of stations in both treatment and control areas was intensively sampled to document the impact of fertilization at various levels of the Lake Mead food chain. Fertilizer additions were designed to temporarily increase epilimnetic total phosphorus concentrations by approximately 20 ug/1 and chlorophyll concentrations by 5-10 ug/1. These increases were achieved, although there were substantial horizontal movements of the fertilized water mass, particularly in 1988. Chlorophyll concentrations peaked about 5 days following fertilization and returned to base-line levels within two weeks. Although maximum chlorophyll levels recorded were about 12 ug/1, thirty-day mean concentrations were below 5 ug/l at all stations. Fertilization produced few adverse water quality impacts and all these were short-lived. Water clarity exhibited temporary decreases at some stations in some years. Taste and odor and disinfection by-product formation potentials exhibited weak correlations with chlorophyll concentrations, but the increases were also short-lived. The fertilizer-induced algal production slightly increased the rate at which epilimnetic concentrations of inorganic nitrogen are typically depleted during the growing season in Lake Mead. The phytoplankton community displayed a complex response to fertilization, with some species clearly increasing their abundance and others showing little or no effect. The proportional abundance of blue-greens decreased following the fertilizer additions. Temporal patterns of algal primary production closely followed those of algal biomass. Size-fractionated production measurements demonstrated that most of the production was occurring in the 20-50 Mm size class, and that this pattern remained essentially unchanged as total production levels increased following fertilization. While interesting questions remain concerning the relative importance of resource levels and other factors, for example predation, in determining shad population abundance, fertilization of the Overton Arm, as undertaken during the present study, clearly did not lead to any "obvious" increase in shad abundance. While a longer-term research program might be able to separate resource-related variance in shad densities from other factors contributing to inter-annual population fluctuations, large-scale artificial fertilization on the scale employed in the present project does not appear to represent a useful management tool for the Lake Mead game fisheries.